1. Field of the Invention
The present invention relates to a method of compression-coding a motion picture signal, and in particular to a method of compression-coding a motion picture signal that has undergone frame thinning. The present invention also relates to an apparatus for carrying out the above-described method.
2. Description of the Related Art
As can be seen in FIG. 1, in the prior art of motion picture compression-coding, the motion picture data is subject to orthogonal transformation at a discrete-cosine-transformer (hereinafter referred to as a DCT) 100, the output of which, i.e., a group of transformation coefficients, is quantized by a quantizer 200. The quantized picture data is variable-length-coded by a variable length coder 300, and the variable-length-coded picture data is stored in a buffer memory 400 for matching transmission speed with the transmission line. In this procedure, a quantization controller 500 carries out optimal quantization control through calculation based on an amount of codes in the immediately preceding frame and a reference amount of the codes predetermined as a desired code amount of one frame.
In this prior art, changes in content of a received picture causes changes in generated picture information. In particular, when rapid changes occur in the picture content, prominent variation occurs in the picture quality of the frames before and after the change.
For example, suppose that the picture content changes to a large extent between the frames of frame number n and frame number (n+1), particularly to the extent that the picture of frame (n+1) have many high spatial frequency components as to cause the code amount to exceed the reference code amount. In this case, a rough or coarse quantization step will be applied to the next frame (n+2), resulting in the code amount of the frame (n+2) prominently to decrease. As a result, a fine quantization step is applied to the frame (n+3), resulting in the picture quality abruptly improved. This phenomenon occurs in the same manner even if there is no change in picture content between the frames of frame number (n+1) and frame number (n+2), once a rapid change takes place at a frame (n+1). Such a phenomenon that picture quality changes even though there is no change in picture content, presents an even more unnatural impression to viewers than a picture of consistently poor quality.
Japanese Patent Laid-open No. 87-85588 discloses a method for solving the same type of problem occurring in an interframe coding method according to which changes in picture content between successive two frames are evaluated, and when these changes are judged to be great, the dynamic range of the quantizer for transformation coefficients is limited narrow, following which the dynamic range is gradually increased to return to a predetermined original breadth. Although abrupt changes in picture quality can be prevented by this method, there remains the problem that significant time is required for the picture quality to restore the level corresponding to the reference code amount.
In the art of coding motion pictures, a technique known as frame thinning is often employed by which the amount of information to be transmitted is reduced by coding motion picture signals for every other frame and, on the receiving side, replaying the coded frames twice. However, when application of the above-described quantization control is attempted for frame-thinned motion picture signals, quantization control is executed based on the code amount generated in the preceding thinned frame, i.e., the frame two frames earlier, thereby exacerbating the above-described problem.